What You Need To Know About Fusion

written by: Sean Fears•edited by: Jason C. Chavis•updated: 5/24/2010

Most people know that stars are thermonuclear furnaces, but there are a lot of details that escape notice, such as how the fusion process (and internal configuration) varies with the star's mass. Why is it that all stars aren’t the same? Before we can get into that, we need to talk about fusion.

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Fusion in the Lab: The ITER Experiment

Humans have had access to the thermonuclear realm since the 1950’s, but the only ways we have to initiate fusion involve creating a high-temperature environment via nuclear fission, or inputting far more energy than we get out of the process to contain and heat matter to plasma via the use of lasers (inertial confinement) or magnetic fields (magnetic confinement). The ITER experiment seeks to break even, producing a net energy surplus. Operational status for the reactor still lies years into the future, though. Stars could be considered gravitational confinement fusion reactors, since they take advantage of vast concentrations of hydrogen to initiate and sustain the fusion process. Even taking into account the necessity of overcoming the natural repulsion of like charges and thermal expansion, this strategy is still the most successful at achieving a reliable, sustainable fusion reaction.

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How Stars Generate Energy

At their most basic, stars exploit E=mc^2 in order to generate energy, fusing each successive element with others already present in order to create heavier ones; the difference between the total masses at each step is given off as energy. Fusion of lighter elements is more efficient (yields greater net energy), and fusing iron or any of the elements past it requires more energy than it produces. The mass of the helium created from the proton-proton (p-p or pp) chain that represent the fusion process of stars less than twice the mass of the Sun is less than one percent lower than the mass of the individual protons involved in the process, yet that difference is enough to generate 26.73 MeV of energy (mega-electron volts, the common unit of particle mass/energy in physics). Compare this with a proton’s rest mass of 937 MeV. To give you a frame of reference, this is about eight times more energy per unit mass than is generated via fission of uranium, which is in turn about 880,000 times more energy per unit mass than is produced via combustion of gasoline. Larger stars undergo a different fusion process, one in which carbon, nitrogen, and oxygen serve as catalysts for the fusion process; however, the total amount of energy produced is almost the same, 26.8 MeV.

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Wasted Energy

There is a catch, though. A significant fraction of this energy, about 35% in the p-p case, is produced as high-energy neutrinos, particles with virtually no rest mass which rarely interact with matter at all. These particles don’t transfer their energy to anything else before leaving the Sun, thus decreasing the net energy generated. The remainder consists of gamma rays and is converted to the wavelengths we see (and those we don’t) during the long trip from the sun’s interior to the surface. How these reactions form and affect stars’ interiors and the typical life cycle of a star are topics we’ll cover in the next article in this series.

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Image Source

Stars, Fusion, and the Main Sequence

Most people know that stars are thermonuclear furnaces, but there are a lot of details that escape notice. How do stars form? Why is it that all stars aren’t the same? What happens at the end of a star's life? Find out the answers to these and other questions in this series!